[llvm-readobj] Extend the output of -elf-section-groups

This diff extends the output of -elf-section-groups 
(llvm style, gnu style is unchanged since it's meant to be 
compatible with binutils readelf) with sh_link and sh_info.
This change will enable us to use llvm-readobj -elf-section-groups 
for testing llvm-objcopy's support for .group sections.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D44280

llvm-svn: 327341